The present invention relates to an adaptive comb filter for deriving a luminance signal (commonly referred to as the Y signal) and a chrominance signal (commonly referred to as the C signal) from a color television signal such as an NTSC color television signal separately in a digital manner by A/D conversion of a color television signal.
A signal sequence S(i,j), for i=1,2,3 . . . m and j=1,2,3 . . . n, obtained by digitizing an analog NTSC signal is shown in FIG. 1. A sampling frequency f.sub.s used to digitize the NTSC signal is usually four times the color subcarrier frequency f.sub.sc. In FIG. 1, a sample S(i,j) is represented by a sum of a Y signal sample Y(i,j) and a C signal sample C(i,j); that is, S(i,j)=Y(i,j)+C(i,j).
For the usual television signal, there is a strong correlation between adjacent samples in both the vertical and horizontal directions in one field. Furthermore, in the NTSC system in which interlaced scanning is performed, the phase of the C signal is reversed every line, that is, the polarity of C changes every two samples as shown in FIG. 2. Taking advantage of this property, it is possible to separate the Y signal and the C signal from the NTSC signal in a digital manner.
FIG. 3 shows the construction of a conventional Y and C signal separating comb filter. In FIG. 3, the filter includes an A/D converter 1 for digitizing the analog NTSC signal on a line 101, a vertical filter 2 for deriving from an output of the A/D converter 1 on a line 102 a line alternating signal H.sub.c (H.sub.c =D+C where D represents a high frequency component of the Y signal in the vertical direction and a low frequency component of the same in the horizontal direction), a bandpass filter 3 receiving the low frequency component in the output of the vertical filter 2 on a line 103 and outputting the C signal on a line 105, a delay element 4 for compensating for delays in the filters 2 and 3, and a subtractor 5 for obtaining a difference between an output of the bandpass filter 3 on the line 105 and an output of the delay element 4 on a line 106.
In operation, the analog NTSC signal on the line 101 is digitized by being sampled by the A/D converter 1 with a predetermined sampling frequency f.sub.s (f.sub.s =4f.sub.sc) to obtain a digital signal series S(i,j) on the line 102. The digital signal series S(i,j) on the line 102 is filtered by the vertical filter 2.
FIG. 4 shows the vertical filter 2 in detail. The filter 2 includes a pair of series-connected line buffer memories 6, each delaying the input signal on the line 102 by a period of one line, an adder 7 for summing the output of the A/D converter 1 and an output of the second of the line buffer memories 6, a multiplier 8 for multiplying an output of the adder 7 by 1/4, a multiplier 9 for multiplying the output of a first of the line buffer memories 6 by 1/2, and a subtractor 10 for obtaining a difference between outputs of the multipliers 8 and 9.
In the vertical filter 2, in order to obtain a line alternating signal H.sub.c (i,j) for the sample S(i,j), samples S(i,j-1), S(i,j) and S(i,j+1) of the output sample series of the A/D converter on lines 108, 104 and 102, respectively, are used. That is, when the sample S(i,j) is selected, sample values in lines just above and below the selected line are used to perform the following operation: EQU H.sub.c (i,j)=-(1/4)S(i,j-1)+1/2 S(i,j) -(1/4)S(i,j+1).
The signal H.sub.c (i,j) appears on the line 103.
As a result, the vertical, low frequency component of the Y signal for the sample S(i,j) is removed. The H.sub.c signal on the line 103 is supplied to the bandpass filter 3 to remove the horizontal low frequency component D of the Y signal, as a result of which a C signal for the sample S(i,j) is obtained on the line 105. At this time, the Y signal is obtained on a line 107 by operations based on the following equation: EQU Y(i,j)=S(i,j)-C(i,j).
In the conventional YC separation comb filter constructed as above, if the video information component of the color television signal varies strongly in the horizontal direction, the energy of the horizontal high frequency component of the Y signal becomes predominant. Therefore, a portion of the horizontal high frequency component of the Y signal still remains in the H.sub.c signal because it is very difficult to realize sharp cutoff characteristics of the bandpass filter 3. Consequently, the YC separation is poor, resulting in the Y signal on the line 107 containing a strong C-signal component mixed therein. Furthermore, since the horizontal high frequency component which is not removed from the Y signal affects the H.sub.c signal for the next subsequent line, the response of the YC separation comb filter to variations of the television signal in the vertical direction is degraded.
Further, an NTSC composite color television signal S is a composite signal composed of a luminance signal Y and a chrominance signal C obtained by quadrature two-level phase modulation of two color difference signals U and V (or I and Q) with a color subcarrier frequency f.sub.sc and can be represented by: EQU S=Y+C=Y+U sin(2.pi.f.sub.sc t)+V cos(2.pi.f.sub.sc t).
Representing the frame frequency, the field frequency and the horizontal scanning frequency by f.sub.F (=30 Hz), f.sub.V and f.sub.H, respectively: EQU f.sub.sc =(455/2)f.sub.H =(455/2).multidot.(525/2).multidot.f.sub.V =(455/2).multidot.525f.sub.F.
Therefore, when the NTSC composite color television signal is sampled with a sampling frequency f.sub.s four times the color subcarrier frequency f.sub.sc, the sample signals are arranged two dimensionally on the image plane as shown in FIG. 5. That is, four samples are derived in one period of the color subcarrier, which are color signals C whose phases are inverted every line.
On the other hand, the PAL composite color television signal P can be represented by: EQU P=Y+U sin(2.pi.f.sub.sc 't).+-.V cos(2.pi.f.sub.sc 't),
where f.sub.sc ' is the color subcarrier frequency. The + sign in the above equation occurs for the even-numbered scanning lines and the - sign occurs for the odd-numbered scanning lines. That is, the V component is inverted every scanning line.
Representing the frame frequency, the field frequency and the horizontal scanning frequency by f.sub.F ' (=25 Hz), f.sub.V ' and f.sub.H ', respectively: EQU f.sub.sc '=(284-1/4+1/625)f.sub.H '=(284-1/4+1/625).multidot.(625/2)f.sub.V '=(284-1/4+1/625).multidot.625f.sub.F '.
That is, the color subcarrier frequency f.sub.sc ' and the horizontal scanning frequency f.sub.H differ by an amount corresponding to 1/4 line. For this reason, the signal sample obtained by sampling the PAL composite color television signal with a sampling frequency f.sub.s ' four times the color subcarrier frequency f.sub.sc ' are arranged two dimensionally as shown in FIG. 6. That is, the phase of the color signal becomes the same after four time periods.
It is necessary, as mentioned above, in a television receiver to correctly separate the luminance signal Y and the chrominance signal C from the composite color television signal containing a color signal which is frequency-multiplied to obtain compatibility with a monochromatic television signal such that the spectrum thereof is frequency-interleaved in the luminance signal region.
FIG. 7 shows the construction of another conventional YC signal separating filter for realizing the above separations. In FIG. 7, reference numerals B-1, B-2, B-3 and B-4 depict a one-line delay circuit, a vertical filter, a horizontal filter and a subtractor, respectively.
The operation of this filter, used for NTSC composite color television signal separation, will be described.
The composite color television input signal on a line B-101, which is sampled in synchronism with the color subcarrier with a sampling frequency f.sub.s= 4f.sub.sc, is arranged two dimensionally as shown in FIG. 5. One sample delay and one line delay are represented by Z transformations Z.sup.-1 and Z.sup.-l, respectively, where: EQU Z.sup.-1 =exp[-j2.multidot..pi..multidot.f/4f.sub.sc ].
Since f.sub.sc =(455/2)f.sub.H, l=910. A line alternating signal containing the color signal is obtained by the vertical filter B-2 on the basis of the input signal on the line B-101, a one-line delayed input signal from a first one of the series-connected one-line delay circuits B-1 and a two-line delayed input signal from the second one-line delay circuit B-1. The transfer function H.sub.v (Z) of the vertical filter B-2 is: EQU H.sub.v (Z)=-(1/4)(1-Z.sup.-l).sup.2.
That is, assuming that the video information content of the television signal is similar between adjacent image elements, the line alternating signal H.sub.c at a coordinate (m,n) in FIG. 5 is derived as: EQU H.sub.c (m,n)=-(1/4) [S(m,n-1)-2S(m,n)+S(m,n+1)].
The signal H.sub.c contains the luminance signal Y. Therefore, the color signal C(m,n), which is a high-frequency component of the signal (H.sub.c (m,n), is separated by the horizontal bandpass filter B-3 from the latter signal. The color signal on a line B-105 is fed to the subtractor B-4 and there used to separate the luminance signal Y(m,n) according to: EQU Y(m,n)=S(m,n)-C(m,n).
The transfer function H.sub.h (Z) of the bandpass filter B-3 may be represented as follows: EQU H.sub.h (Z)=-(1/32) (1-Z.sup.-2).sup.2 (1+Z.sup.4).sup.2 (1+Z.sup.-8).
As to the operation of the filter for a PAL composite color television signal, the composite color television input signal sampled in synchronism with the color subcarrier with sampling frequency f.sub.s '=4f.sub.sc contains the color signal whose phase is arranged as shown in FIG. 6. That is, the color signal phase becomes the same every four line periods and, for even numbered lines, the color signal component whose color subcarrier phase changes by 180.degree. corresponds to a preceding line while, for odd numbered lines, it corresponds to a subsequent line. Therefore, in order to obtain the PAL four-line alternating signal H.sub.c ' corresponding to the NTSC line alternating signal with the vertical filter B-2, the operation thereof is switched between odd- and even-numbered lines. That is for odd numbered lines: EQU H.sub.c '(m,2n-1)=(1/2) [P(m,2n-1)-P(m,2n)],
while for even numbered lines: EQU H.sub.c '(m,2n)=(1/2) [P(m,2n)-P(m,2n-1)].
In other words, the alternating signal is derived by using a pair of vertically paired sample points on paired lines and the color signal C(m,n) is obtained from the horizontal bandpass filter B-3.
In this conventional luminance/chrominance separating filter, the fixed vertical filter and the horizontal filter are combined, with the assumption that adjacent image elements in the sampled television signal are similar to each other. However, if this is not the case, the luminance signal and the chrominance signal will interfere with one another, particularly in a region in which the luminance and chrominance of the image vary strongly, resulting in distortions in the reproduced image such as color fading and indistinct boundaries.
Another digital filter of the prior art used for the NTSC system is shown in FIG. 8. In this figure, C-1 is a vertical filter, C-2 is a band-pass filter, and C-3 is a subtractor. The vertical filter C-1 is composed of a vertical filter operation circuit C-10 and one-line delay circuits C-11 and C-12. C-101 to C-106 indicate signal lines.
Here, it is presumed that the signal S indicated in the equation above is input to the signal line C-101 as the digitized signal S as shown in FIG. 1. Thus, the signal S(m,n) is present on the signal line C-102 as the output of the one line delay circuit C-11, while the signal S(m,n-1) is present on the signal line C-103 as an output of the one-line delay circuit C-12.
The operation circuit C-10 executes the operations of H.sub.c (m,n)=-(1/4)[S(m,n-1)-2S(m,n)+S(m,n+1)] and outputs H.sub.c (m,n). Here, the signals S(m,n-1), S(m,n) and S(m,n+1) are on adjacent lines in the vertical direction, and therefore the values of Y, U and V in the equation above must be very approximate in general.
Values of Y and C are assumed respectively to be (Y.sub.0 +.DELTA.Y.sub.1,C.sub.0 +.DELTA.C.sub.1), (Y.sub.0,C.sub.0) and (Y.sub.0 +.DELTA.Y.sub.2,C.sub.0 +.DELTA.C.sub.2) for the signals of S(m,n-1), S(m,n) and S(m,n+1). In this case, the values .DELTA.Y.sub.1, .DELTA.Y.sub.2, .DELTA.C.sub.1 and .DELTA.C.sub.2 are generally very small. Accordingly, H.sub.c (m,n) is as follows: EQU H.sub.c (m,n)=-(1/4)[(.DELTA.Y.sub.1 +.DELTA.Y.sub.2)-4C.sub.0 +(-.DELTA.C.sub.1 -.DELTA.C.sub.2)] EQU =C.sub.0 +(1/4)(.DELTA.C.sub.1 +.DELTA.C.sub.2)-(1/4)(.DELTA.Y.sub.1 +.DELTA.Y.sub.2).
The signal S(m+1,n+1) follows the signal S(m,n+1) on the signal line C-101. In this case, the output H.sub.c (m+1,n) of the operation circuit C-10 can be expressed as: EQU H.sub.c (m+1n)=-(1/4)[(.DELTA.Y.sub.1 '+.DELTA.Y.sub.2 ')-4C.sub.0 '+(-.DELTA.C.sub.1 '-.DELTA.C.sub.2 ')] EQU =C.sub.0 '+(1/4)(.DELTA.C.sub.1 '+.DELTA.C.sub.2 ')-(1/4)(.DELTA.Y.sub.1 '+.DELTA.Y.sub.2 ')
In these equations, the values .DELTA.C.sub.1, .DELTA.C.sub.2, .DELTA.C.sub.1 ',.DELTA.C.sub.2 ', .DELTA.Y.sub.1, .DELTA.Y.sub.2,.DELTA.Y.sub.1 'and .DELTA.Y.sub.2 ' are generally very small. Values of Y and C are assumed respectively to be (Y.sub.0 '+.DELTA.Y.sub.1 ',C.sub.0 '+.DELTA.C.sub.1 '), (Y.sub.0 ',C.sub.0 ') and (Y.sub.0 '+.DELTA.Y.sub.2 ',C.sub.0 +.DELTA.C.sub.2 ') for the signals of S(m+1,n-1), S(m+1,n) and S(m+1,n+1). Namely, the signal S applied to the vertical filter C-1 includes a digitized luminance signal Y and a chrominance signal C, and it is clear from the two equations immediately above that the luminance signal Y is strongly attenuated with respect to the output of the operation circuit C-10.
Delays of one sampling period and of one line period are respectively represented as Z.sup.-1 and Z.sup.-l using a Z transform. EQU Z.sup.-1 =exp[-j2.pi.f/f.sub.s ]
and l=910 can be obtained from the expression for S above.
When the signal S is present on the signal line C-101, the signal S delayed by one line period and the signal S delayed two line periods are present on the signal lines C-102 and C-103, respectively. In this case, the transfer function H.sub.v (z) of the vertical filter can be expressed as follows: EQU H.sub.v (z)=-(1/4)(1-2Z.sup.-l +Z.sup.-2l) EQU =-(1/4)(1-Z.sup.-l).sup.2 .
The bandpass filter C-2 is provided to further attenuate the signal Y still remaining in the output of the vertical filter C-1. This bandpass filter C-2 is a bandpass digital filter designed to pass signals U and V having a frequency band around the center frequency f.sub.sc.
The transfer function H.sub.B (z) of the bandpass filter C-2 is such as to satisfy the relation: EQU H.sub.B (z)=-(1/32)(1-Z.sup.-2).sup.2 (1+Z.sup.-4).sup.2 (1+Z.sup.-8).
The output of the bandpass filter C-2 can be considered as the digital chrominance signal C, and thus S-C=Y can be obtained at the output of the subtractor C-3.
A similar filter intended for use with the PAL system can be constructed taking into account the signal characteristics shown in FIG. 6. Namely, operations expressed by the following equation are carried out using an upper two adjacent lines and lower two adjacent lines next to a given line, and result is applied to the signal line C-104: EQU H.sub.cp (m,2n)=-(1/4)[S(m,2n-2)-2S(m,2n) +S(m,2n+2)].
In this prior art system, particularly the vertical filter C-1 is configurated on the assumption that the values of .DELTA.Y.sub.1, .DELTA.Y.sub.2, .DELTA.Y.sub.3, .DELTA.Y.sub.4, .DELTA.C.sub.1, .DELTA.C.sub.2, .DELTA.C.sub.1 ' and .DELTA.C.sub.2 ', etc., are small. Therefore, the system of the prior art is accompanied by the disadvantage that the vertical filter will not properly operate as a filter in a region where changes in luminance and chrominance of the picture image portion corresponding to the adjacent horizontal scanning line (such as a straight line in the horizontal direction) are distinctive, and thereby impurities of color and other disturbances in the picture unavoidably occur.
The present invention further relates to a subnyquist sampling filter for recovering a high quality picture signal from a quantized picture signal sampled at a sampling frequency equal to or lower than the nyquist frequency, for example, for high quality digital transmission of television picture signals.
An existing subnyquist sampling method wherein a signal is sampled with a sampling frequency lower than the nyquist frequency, will be explained referring to FIG. 9. FIG. 9 shows the allocation of a picture signal extracted by subsampling from a series of existing picture signals sampled in the form of a square lattice. In this figure, the maximum frequency of the picture signal is f.sub.v, the nyquist sampling frequency of the picture signal is f.sub.s, the subnyquist sampling frequency is f.sub.s /2 where f.sub.s .gtoreq.2f.sub.v. FIG. 9 also shows a two-dimensional allocation of a subnyquist sampled picture signal which has been obtained by phase inversion for each line period of horizontal scanning for a series of picture signals where the picture signal is sampled in the form of a square lattice on a two-dimensional allocation with the nyquist sampling frequency f.sub.s. As shown in FIG. 9, white circles indicate sample picture elements extracted by subnyquist sampling, while black circles indicate interpolated picture elements recovered by interpolation in order to compensate for the extracted picture elements.
As described above, a subnyquist sampling method is effective as a method of highly efficient coding to reduce the amount of data during transmission and recording by reducing the number of samples of the picture signal. A prefilter which suppresses loop noise produced by the subnyquist sampling and an interpolation filter which results in less interpolation noise during recovery are required in the subnyquist sampling filter.
FIG. 10 is a block diagram indicating an example of a conventional subnyquist sampling system. FIGS. 11 and 12, respectively, are block diagrams of a prefilter and an interpolation filter used in the subnyquist sampling system of FIG. 10, In these figures, D-2 and D-10 are A/D converters and D/A converters, respectively, D-4 is a prefilter, D-6 is a subsampler, D-8 is an interpolation filter, D-12 and D-18 are line delay circuits, D-14 and D-16 are dot delay circuits, D-20 is a low-pass filter, D-22 and D-25 are subsample line delay circuits, D-27 is an average value interpolation filter, and D-29 is an interpolation switch.
Operations of the subnyquist sampling system of the prior art shown in FIG. 10 will be explained. An analog input picture signal D-1 is sampled at the nyquist sampling frequency f.sub.s by the A/D converter D-2 and converted into a digital input picture signal D-3. This digital input picture signal D-3 is subjected to suppression of high frequency components to be folded back on the low frequency components by the prefilter D-4 and is then subnyquist sampled as shown in FIG. 9 in the subsampler D-6. The subnyquist sampled subsample output picture signal D-7 is subjected to interpolation of picture elements extracted by the interpolation filter D-8, converted to a digital recovered picture signal D-9, and then converted to an analog recovered picture signal D-11 by the D/A converter D-10.
The operations of the prefilter shown in FIG. 11 will now be described. Here, a digital input picture signal D-3 is indicated by S(m,n) corresponding to the picture element location When S(m,n+1) indicated in FIG. 9 is input to the digital input picture signal D-3 through the line delay circuits D-12 and D-18 and the dot delay circuits D-14 and D-16, a total of five picture signals S(m+1,n), S(m,n), S(m-1,n) and S(m,n-1) (corresponding to D-13, D-15, D-17 and D-18, respectively) are output simultaneously. The low-pass filter 20 carries out the following equations on the picture signals D-3, D-13, D-15, D-17 and D-19, and also outputs the signal P(m,n) indicated below as the prefilter output picture signal D-5 for S(m,n). EQU P(m,n)=(1/8)[S(m,n-1)+S(m-1,n)+4S(m,n) EQU +S(m+1,n)+S(m,n+1)].
The signal P(m,n) is an output of a two-dimensional low-pass filter D-20 which suppresses the high frequency components of the signal S(m,n) which cause loop noise.
The operations of the interpolation filter shown in FIG. 12 will be explained hereinafter. The output picture signal P(m,n) of the prefilter D-4 is reduced to a half in the amount of picture data it contains because of the skipped samplings. A series of samples of the output picture signal D-7, which is the output of subsampler D-6, are sequentially output as P(m,n-1), . . . P(m-1,n), P(m+1,n), . . . , P(m,n+1), . . . on a time-sequential basis. Accordingly, the picture signal extracted by the subsampler D-6 is recovered by the interpolation filter D-8 as explained below. The subsample line delay circuits D-22 and D-25 and the dot delay circuit D-14 simultaneously output the four picture signals of P(m-1, n), P(m+1,n) and P(m,n-1) (corresponding to D-23, D-24 and D-26) with such a timing that the signal P(m,n+1) of the subsample output picture signal D-7 is input. The average value interpolation filter D-27 recovers the interpolation picture signals Q(m,n) (D-28) indicated below corresponding to the element P(m,n) by the following processing from the picture signals mentioned above: EQU Q(m,n)=(1/4)[P(m,n-1)+P(m-1,N) EQU +P(m+1,n)+P(m,n+1)].
The interpolation switch D-29 alternately selects the subsample output picture signal D-23 and interpolation picture signal D-28 and obtains the recovered picture signals R(m,n) (D-9) as the time series . . . P(m,n-1), . . . Q(m-2,n), P(m-1,n), Q(m,n), P(m+1,n), . . . , P(m,n+1), . . . .
The prefilter and interpolation filter in the above subnyquist sampling system are capable of suppressing loop noise by subnyquist sampling, but, on the other hand, they reduce the two-dimensional frequency response of the original picture signal to a half. Namely, the existing subnyquist sampling system has a disadvantage that the resolution in each direction of the picture is lowered to about a half of the resolution provided in the original signal. Moreover, it is also possible to employ an interpolation filter which horizontally interpolates a lower frequency component in the horizontal direction and vertically interpolates a higher frequency component in the vertical direction and is a prefilter which is adaptive to such interpolation filter. However, this technique is also accompanied by a disadvantage that only a frequency response in two-dimensional space can be obtained as in the case of the existing subnyquist sampling system for frequency components which are high simultaneously in the vertical and horizontal directions.